1. Field of the Invention
The present invention relates to semiconductor fabrication, and more particularly, to a method for cleaning the contact holes of semiconductor devices to eliminate unwanted impurities and deposits introduced during the contact hole formation process.
2. Description of the Related Art
Contact holes are apertures formed through layers of a semiconductor device for the purpose of filling them with conducting material such as aluminum and thereby electrically coupling one layer to another layer. By way of example, such a contact hole can be formed to connect an upper electrode with a lower electrode in a multilayered unit cell structure of a semiconductor device.
Contact holes are conventionally formed using photo-etching techniques that are well known in the art. Such a process has several drawbacks, however. First, the chemical reaction of the etched region and the ambient oxygen in the atmosphere causes a native oxide film to grow on the lower side of the contact hole. Second, a chemical reaction between the carbon of the photoresist mask used during lithography with the reaction gas used in the dry-etching process causes a polymer to form on the sidewalls of the contact hole. These unwanted deposits deteriorate the electrical characteristics of the semiconductor device. Third, impurities such as copper (Cu), Gold (Au) and other materials are known to exist in the contact hole after formation for a variety of reasons. Such deposits are known to cause failure of the semiconductor device.
To address these problems, a cleaning process is carried out on the contact holes in order to remove the native oxide film, the polymer, or impurities such as Cu and Au. In the conventional cleaning process, the semiconductor wafer is bathed in a first cleaning process at 120.degree. C. in a cleaning composition composed of H.sub.2 SO.sub.4 and H.sub.2 O.sub.2 in respective weight percents of 80/20 or 85/15. The profile of the inside wall of the contact hole is maintained in this first cleaning process. Then, the wafer is bathed in a second cleaning process at 25.degree. C. for 45-90 seconds in a cleaning composition composed of H.sub.2 SO.sub.4 and H.sub.2 O.sub.2 in respective weight percents of 99.5/0.5.
Because of the small size of the contact holes and the surface tension of the cleaning solutions used, it is difficult to introduce the cleaning solution into the lower end of the contact hole. This becomes even more of a problem as device sizes are reduced due to improvements in lithographic techniques. Consequently, despite these cleaning processes, the native oxide film and other impurities still remain on the lower side and sidewalls of the contact hole. This results in a degradation of the electrical characteristics of the semiconductor devices.
Accordingly, the need exists for a better method for removing unwanted deposits and impurities from contact hole formation during the manufacture of semiconductor devices.